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Lab View Nano VNA system


John Galbreath 2021/08/09 20:20

I have read the manual & watched Joe Smith's You Tube tutorials and I still have questions.

1.  Calibration Error - Always on even if sweep range is within calibration range - OK to ignore ?

2.  Resistance value is negative sometimes - OK to ignore ?

3.  How to move cursor B to min or max using pushbuttons ?

4.  P - P (hz)  on crystal parameter - what is P-P ?

5.  page 30 - missing m

6.  page 94 - beach towel phrase does not make sense

7.  Will 2 SPDT PIN diode switches (plus SPST relay for status of relay) work for a transfer relay ?
Could you share your USB control details ?

alepplew 2021/08/10 01:42

Something is wrong with your calibration then. When within the original calibration range, error must be off.
Maybe you activated the wrong stimulus range. There are two different ranges, the first one center/span, the second min/max. If you click on either of these titles or numbers, you activate that range. Only visible by the freq scale in the graphs.
Futhermore, when you load a calibration, it activates the min/max range that was active during that calibration.

Jim Allyn - N7JA 2021/08/10 13:45

On Mon, Aug 9, 2021 at 08:20 PM, John Galbreath wrote:

>
> I have read the manual

I haven't kept upon all the documents here.  Which manual are you talking about?  If you can provide a link it would allow me to see things in context.

Negative resistances can occur with active devices: transistors, ICs, etc.

I would assume P - P is peak to peak.

Jim Allyn - N7JA 2021/08/11 12:24

Responding to John Galbreath's off-list message: John, it is best to keep the exchange on the list.  If you keep it on-list, everybody will know what you asked and what response you got to it.  That way **everybody** can benefit from whatever answers you might get.  Also, suppose I responded off-list and gave you an incorrect answer.  Nobody else would see it, therefore, nobody would correct my incorrect answer.  So please keep things on-list.  Thanks for sending the manual.

P-P, in this instance, refers to the amplitude of the noise displayed in Figure 77.  Look at the graph in Figure 77.  The highest peak value of the noise is -0.01 dB, and the lowest peak value is just almost -0.05 dB.  The peak to peak (P-P) value is therefore the difference between these two peaks, -0.01 - -0.05, or just about 0.04 dB P-P.

I would say that with possibly a rare exception,  you should not ignore the Calibration Error indicator.  In this case, the Calibration Error is what is likely what's producing the negative resistance readings you see.  I'm assuming the magnitude of the negative resistance you are seeing is relatively small?  A slight calibration error could cause that.

The reference to the beach towel, "If you have a lot of air flow in the room, my standard practice it the beach towel" ["it" should be "is"] suggests wrapping the device in a beach towel to insulate it from rapid temperature changes in the room.  If there is a draft of air flowing through the room, the temperature could vary several degrees or more in a short period of time.  Wrapping the device in a towel will shield it from those drafts and keep the average temperature more stable.

I generally calibrate my VNAs specifically for the task at hand, and immediately prior to taking the measurement.  Readings will vary with temperature, battery voltage, and so on, so a fresh cal is always best.  When you are measuring something like a crystal, a broadband calibration is useless.  Suppose you have done a cal on your VNA from 5 MHz to 25 MHz,  and you wish to measure a 12 MHz crystal.  You will NOT get good results.  Yes, 12 MHz is within the 5 to 25 MHz calibration range, but the VNA sweeps frequency in steps.  The step size of the sweep means that no measurements at all are likely to have been made at the crystal frequency.  The software will probably try to extrapolate from the cal data it has, and that would be fine for an LC filter or something that changes characteristics slowly as the frequency changes.  But when all the action takes place within a few kiloHertz, a broadband calibration doesn't give it enough data to extrapolate from, and any readings you get will be useless.

Hope this helps.

John Galbreath 2021/08/11 18:48

My first educated guess about P - P was the difference between the highest peak at the series resonant frequency less the lowest valley at the parallel resonant frequency, however none of my crystal parameter tests would agree with this hypothesis.  I placed A cursor at series and B cursor at parallel and none of this data  came anywhere close to the P - P value and I cannot find any explanation in the manual or You Tube tutorials.  The crystal tested in the attachments had 18 Khz difference between series and parallel resonant frequencies however the value shown for P -P is 5 Hz.  Several other different crystals had similar results.

John Galbreath 2021/08/12 08:40

Thanks.  I clicked wrong reply button.  Sorry about that.

I should have stated that the P - P (Hz) of which I questioned is on page 83, figure 66, not figure 77 so my question remains unanswered.

According to video tutorial, calibration is not necessary for crystal characterization, however it is necessary to "normalize" level by inserting short into fixture in place of crystal, then set "reference", then set "CHn-REF".

When I need to calibrate (not crystals), I always set sweep range inside calibration range yet I always get red light calibration error, so the cause is probably operator error, or incompatibility of the firmware in my clone (from AURSINC).  My negative resistance readings are -137 and up.

-John G
W6PUY

Joe Smith 2021/08/12 10:40

After the constant direct emails from you, which help no one but yourself, I had added you to the SPAM filter.   I certainly understand your not wanting to make your posts public but at the same time, I only have so much time and it makes little sense spending it on individuals in private discussions.   While I'm sure you see yourself as my customer, let me be clear, you're not.  The software was not written for you, but everyone is certainly free to use it.   Personally, I recommend people not waste time with it and stay with the open source software that is available.

1.  Calibration Error - Always on even if sweep range is within calibration range - OK to ignore ?

With the sweep range inside the calibration range, there should be no error.   Without my crystal ball, I have no idea what you are doing or seeing.  If there is a problem with the software or something in your procedure and you would like to try and work through it, the easiest thing to do would be to make a video, or document each step.  Start with powering on the VNA.  It's up to you to provide enough detail so others can attempt to replicate what you are seeing.

"... however it is necessary to "normalize" level ..."
While I have covered it a few times, it's worth pointing out that in many of my videos and posts, I won't take the time to calibrate the VNA.   In some cases I may seem flippant with my attitude towards the subject.  There are a few reasons for this.  Many VNAs can output good enough data without calibration to get me close enough.  In many cases, I am able to get by with using normalization.  There is also wear and tear on the standards.  In my case, I do not own a set of metrology grade standards and would have little use for them for my home projects.  Still, even my home made standards have a limited life.  It also takes time to perform a proper calibration which brings up my last point.  I am lazy.

If I do need to measure my VSWR down to thee places past the decimal (had a radio hobbyist tell me that), I will normally make all of my measurements with the VNA uncalibrated.  If everything makes sense and I can see the numbers coming out to what I expect them to be, then I take the time to calibrate and make my final measurement.

My first VNA (HP8754A) has no way of performing a SOLT and only has normalizaion (with an HP8501A).  That's actually the reason that I first wrote the software was to allow me to calibrate that unit.   It's up to the user to understand what they require to make their measurement.   Not just the standards, how to use them but also the VNA, cables, torque wrenches, adapters.   The best thing about these low cost VNAs is your out little cash if you screw something up.  They are a very good starting point for those wanting to learn some of the basics.

2.  Resistance value is negative sometimes - OK to ignore ?

I assume the user can answer that.

3.  How to move cursor B to min or max using pushbuttons ?

It should have been obvious that both cursors are effected when selecting the buttons you mention.  FYI, I may change how these work in the future.  For example,  Max I've thought about moving A to the highest and B to the second highest.  Maybe adding a Max/Min button where A goes to max and B to minimum.  For now, I don't have a use for any of it but will say the software has certainly evolved since I first wrote it for my HP8754A.

4.  P - P (hz)  on crystal parameter - what is P-P ?   "...I cannot find any explanation in the manual or You Tube tutorials."

First, the manual is certainly not all inclusive.  It covers some basics on how to use the software.   This is one difference between a home project and what one may expect from a purchased commercial product.

In the case of your specific question, once again I thought this was rather obvious and because these numbers are not used as part of the typical calculations, I chose to not spend any time going over them.   All of the statistics are for the histogram to the right.  Look at your screen shot and look at the histogram.  Max is towards the right, Min to the left.  P-P(KHz) is Max - Min.  Standard deviation is taking into account all of the data collected.  Visually, we can see it has a gaussian distribution and the Mean works out close to the center.

5.  page 30 - missing m

I suspect there are many small errors like this but most readers are able to understand what is being described.  I'm actually surprised that you wouldn't have mentioned the use of "K" vs "k" in the software on the page you posted.   That's something I really should take the time to correct.  I am really bad when it comes to case and units but the context normally saves me.

6.  page 94 - beach towel phrase does not make sense

I thought it was obvious and another member did a fine job covering it.

7.  Will 2 SPDT PIN diode switches (plus SPST relay for status of relay) work for a transfer relay ?
Could you share your USB control details ?

That's really up to the designer to come up with what ever circuit meets their criteria.  The manual covers the use of a transfer relay.  What specifically were you not understanding?

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